mg5decay useage with lots of decay chains

I'm currently studying a process that has many decay chains, and I initially used the following syntax to simulate:

p p > A B, (A > C D, C > E F, D > G H), (B > I J, I > K L , J > M N)

Then I found that it was very time-consuming (as long as 15 hours!!!) to finish the simulation even though I only generated 1M events.

Then I heard that I could use "mg5decay" tool to speed up, the flow was something like:
1. generate 1M events by "p p > A B, (A > C D), (B > I J)" and stop here.
2. Then use mg5decay to add the left decay chain, i.e., C > E F, D > G H etc
3. As mg5decay will automatically run pythia and delphes, we are then ready to do analysis.

The idea is very clear, but I don't know how to get the second step done. If you could please help, I would really appreciate it.

We do have a folder inside MG with name mg5decay. Previously we though it would help solve our situation, but later we found it is madspin that does what we are seeking for. But thank you any way for the comment.